Hydrophobic catalyst layer for polymer electrolyte fuel cell and method of producing the same, and polymer electrolyte fuel cell and method of producing the same

ABSTRACT

Provided is a hydrophobic catalyst layer for a polymer electrolyte fuel cell to which hydrophobicity is imparted so that the dissipation property of produced water is improved and which simultaneously has an increased effective surface area and an increased utilization ratio of a catalyst, and a method of producing the same. The catalyst layer for a polymer electrolyte fuel cell includes a catalyst obtained by reducing a platinum oxide, a hydrophobic agent, and a proton conductive electrolyte, wherein the hydrophobic agent is mainly composed of alkylsiloxane. An Si compound containing a hydrophobic substituent is brought into contact with a platinum oxide to subject the Si compound to hydrolysis and a polymerization reaction by the catalytic action of the platinum oxide, and then it is reduced, thereby obtaining a hydrophobic catalyst layer carrying an alkylsiloxane polymer.

This application is a continuation of International Application No.PCT/JP2006/309356, filed Apr. 28, 2006, which claims the benefit ofJapanese Patent Application No. 2005-132957, filed Apr. 28, 2005.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a hydrophobic catalyst layer for apolymer electrolyte fuel cell and a method of producing the same, and apolymer electrolyte fuel cell and a method of producing the same.

2. Description of the Related Art

A polymer electrolyte fuel cell is expected to be an energy generatingapparatus in the future because the cell has high energy conversionefficiency, and is clean and quiet. Investigation has been recentlyconducted into the application of the polymer electrolyte fuel cell tonot only a power source for an automobile, a domestic generator, or thelike but also a power source for, for example, a small-size electricalapparatus such as a portable phone, a notebook personal computer, or adigital camera because the polymer electrolyte fuel cell has a highenergy density and can operate at a low temperature. The polymerelectrolyte fuel cell has been attracting attention because it may bedriven for a long time period as compared to a conventional secondarybattery.

The polymer electrolyte fuel cell has an advantage in that it can bedriven even at an operating temperature of 100° C. or lower. On theother hand, the polymer electrolyte fuel cell has a problem in that thevoltage of the cell gradually reduces with the lapse of an electricitygenerating time, and finally the cell stops generating electricity.

Such problem results from a so-called “flooding phenomenon” in whichwater produced as a result of a reaction resides in gaps of a catalystlayer, and water clogs the gaps in the catalyst layer to inhibit thesupply of a fuel gas as a reactant, so that an electricity generationreaction is stopped. Flooding is apt to occur particularly in a catalystlayer on a cathode side where water is produced.

In addition, a reduction in size of the entire system is essential tothe practical use of the polymer electrolyte fuel cell for a small-sizeelectrical apparatus. In particular, in the case where the fuel cell ismounted on a small-size electrical apparatus, not only the size of theentire system but also the size of the cell itself must be reduced.Accordingly, a mode (air breathing) in which the air is supplied from anair hole to an air electrode through natural diffusion without the useof a pump, blower, or the like is considered to be promising.

In such case, the produced water is discharged to the outside of thefuel cell only by natural evaporation, so that the produced waterresides in a catalyst layer to cause flooding in many cases.Accordingly, imparting hydrophobicity to the catalyst layer to improvethe dissipation property of the produced water is considered to be animportant factor on which the stability of the performance of the fuelcell depends.

A conventionally known method of making a catalyst layer hydrophobicinvolves mixing a fluorine resin-based fine particle powder made ofpolytetrafluoroethylene (PTFE) or the like as a hydrophobic agent with asolvent or a surfactant upon formation of the catalyst layer.

In addition, there have been proposed a method involving imparting theconcentration distribution of hydrophobicity to the thickness directionof a catalyst layer to improve the dissipation property of producedwater additionally (Japanese Patent No. 3245929), and a method involvingmaking a part to which hydrophobicity is imparted maldistributed in thesurface of a catalyst layer (Japanese Patent Application Laid-Open No.2004-171847).

In addition, Japanese Patent Application Laid-Open No. 2001-76734discloses a method of mixing fine particles composed ofdimethylpolysiloxane in addition to fluorine-based resin fine particles.Japanese Patent Application Laid-Open No. 2001-76734 describes that theparticle size of each of the hydrophobic fine particles is equal to thatof a carbon carrier particle, and is preferably 10 μm or less.

Meanwhile, Japanese Patent Application Laid-Open No. 2006-49278 andJapanese-translated version's National Publication No. 2001-51959 eachdisclose a method of forming a catalyst layer for a fuel cell by meansof a sputtering method or an ion plating method.

As described above, an approach of forming a catalyst layer by means ofa vacuum film formation process such as a sputtering method has beenrecently developed. A conventional method involves: mixing catalystparticles, an electrolyte, and a solvent to prepare slurry; and mixingthe slurry with hydrophobic particles to make the slurry hydrophobic. Onthe other hand, such production method as described in each of JapanesePatent Application Laid-Open No. 2006-49278 and Japanese-translatedversion's National Publication No. 2001-51959 does not involve mixinghydrophobic fine particles upon formation of a catalyst layer. That is,hydrophobicity is not imparted by means of a mixing method.

SUMMARY OF THE INVENTION

Hydrophobic fine particles that have been conventionally used such asthose described in Japanese Patent Application Laid-Open No. 2001-76734have neither conductivity nor proton conductivity, and are mixed withand randomly dispersed into catalyst particles, an electrolyte, carrierparticles, and the like.

Accordingly, when conventional hydrophobic fine particles are used, thehydrophobicity of a catalyst layer improves, but there arises a problemin that part of the hydrophobic fine particles enter a gap betweenadjacent catalyst particles or between a catalyst and the electrolyte,so that a proton adsorption area of the surface of the catalyst, thatis, an effective surface area reduces, and hence the utilization factorof the catalyst reduces.

Furthermore, the diameter of each of fluorine resin-based hydrophobicfine particles that have been conventionally and generally used in awide variety of applications is about 100 nm to several hundreds ofmicrometers, and the diameter of a secondary agglomerate particle isadditionally large. Even the diameter of each of the hydrophobicparticles described in Japanese Patent Application Laid-Open No.2001-76734 is about 10 μm which is almost equal to that of a carboncarrier particle.

Since the conventional hydrophobic particles each have such particlesize, it is impossible in principle to make the inside of a gap having asize of less than 100 nm in a catalyst layer (hereinafter referred to asthe “micro-gap”) hydrophobic. In this case, the inside of the micro-gapremains hydrophilic. Accordingly, when the outside of the micro-gap ismade hydrophobic by a large hydrophobic particle, produced water istrapped in the micro-gap in some cases. As a result, there arises aproblem in that local flooding occurs in the micro-gap to reduce theutilization factor of a catalyst.

In addition, a conventional hydrophobic agent is granular. Accordingly,when the size of each of hydrophobic fine particles is almost equal tothat of each of gaps, the gaps are clogged with the hydrophobic fineparticles, and the gas permeability of a reactant gas reduces.Accordingly, a reaction in each of the gaps stops. As a result, therearises a problem in that the utilization factor of a catalyst reduces.

As described above, in the prior art there is a problem of imperfectlyimparting hydrophobicity to a catalyst layer, and simultaneouslyreducing the utilization factor of a catalyst occurs.

As a result, conventionally, as compared to the case where nohydrophobicity is imparted, the voltage of a fuel cell in ahigh-current-density region increases, but the voltage of the fuel cellin a low-current-density region reduces.

Accordingly, a technique for achieving compatibility between theimpartment of hydrophobicity to a catalyst layer and an increase inutilization factor of a catalyst has been requested.

Meanwhile, when a catalyst layer is formed by means of a sputteringmethod or the like as shown in each of Japanese Patent ApplicationLaid-Open No. 2006-49278 and Japanese-translated version's NationalPublication No. 2001-51959, the catalyst layer cannot be formed bymixing hydrophobic fine particles unlike the prior art, so thathydrophobicity cannot be imparted by means of a conventional mixingmethod. In this case, there arises the following problem: even whenconventional fluorine resin-based hydrophobic fine particles are appliedto a catalyst layer after the formation of the catalyst layer, most ofthe diameters of the pores of the catalyst layer are about severalhundreds of nanometers and smaller than that of each of the hydrophobicfine particles, so that the hydrophobic fine particles are not dispersedinto the catalyst layer, and hydrophobicity cannot be effectivelyimparted to the inside of the catalyst layer.

The present invention has been accomplished in view of suchcircumstances as described above, and provides a hydrophobic catalystlayer for a polymer electrolyte fuel cell, in which hydrophobicity iseffectively imparted to also a micro-gap inside the catalyst layer, andsimultaneously an effective surface area and an increased utilizationfactor of a catalyst are increased. In addition, the present inventioncan provide hydrophobicity and an increase in effective surface area foreven a catalyst layer formed by means of a sputtering method.

In addition, the present invention provides, at a low cost, a polymerelectrolyte fuel cell having stable electricity generation property byusing the above hydrophobic catalyst layer to which hydrophobicity isimparted.

The present invention has been accomplished in order to solve theabove-mentioned problems.

That is, the present invention provides a hydrophobic catalyst layer fora polymer electrolyte fuel cell, including: a catalyst, a hydrophobicagent, and a proton conductive electrolyte, wherein the catalyst is adendritic-shaped catalyst obtained by reducing a platinum oxide, whereinthe hydrophobic agent is composed of a compound having an Si atom, an Oatom, and a hydrophobic substituent, and wherein a ratio Si/Pt of anumber of Si atoms in the hydrophobic agent to a number of Pt atoms inthe catalyst is in a range of 0.15 or more and 0.25 or less.

The hydrophobic agent is preferably composed of a siloxane polymerhaving a hydrophobic substituent.

In addition, the hydrophobic agent is preferably composed ofalkylsiloxane.

Further, the present invention provides a method of producing ahydrophobic catalyst layer for a polymer electrolyte fuel cell,including the steps of: bringing an Si compound containing a hydrophobicsubstituent, which causes a hydrolytic reaction owing to a catalyticaction of a platinum oxide to form a polymerizable group, into contactwith the platinum oxide; subjecting the Si compound to a polymerizationreaction in a vicinity of the platinum oxide to form a hydrophobic agenton a surface of the platinum oxide; and reducing then the platinumoxide.

The Si compound is preferably at least one or more compounds selectedfrom the group consisting of 2,4,6,8-tetraalkylcyclotetrasiloxane,1,1,1,3,3,3-hexaalkyl-disilazane, monoalkylsilane, dialkylsilane, andtrialkylsilane, or a mixture thereof.

Further, the present invention provides a polymer electrolyte fuel cellincluding the hydrophobic catalyst layer.

According to the present invention, the hydrolysis and polymerizationreaction of an Si compound containing a hydrophobic substituent areinitiated on the surface of a platinum oxide so that a hydrophobic agentis formed in the inside of each pore of a catalyst layer including theinside of a micro-gap. After that, the oxide is reduced, whereby theutilization factor of a catalyst and the dissipation property ofproduced water can be simultaneously improved.

The hydrophobic agent is formed from an Si compound molecule smallerthan a micro-gap in each pore of the catalyst layer by thepolymerization reaction, so that the impartment of hydrophobicity to theinside of a micro-gap having a size of 100 nm or less which has beenconventionally difficult is attained.

In addition, the present invention provides, at a low cost, a polymerelectrolyte fuel cell having stable characteristics by using the abovecatalyst layer with improved dissipation property of the produced water.

Further, the present invention can provide a polymer electrolyte fuelcell having additionally stable characteristics at a low cost.

Furthermore, according to the present invention, a contact area betweena catalyst and an electrolyte, that is, an effective surface area thatcan contribute to a catalytic reaction can be increased, whereby theutilization factor of the catalyst can be increased.

As a result, the simultaneous achievement of the impartment ofhydrophobicity and an increase in utilization factor of a catalyst whichhas been conventionally difficult is enabled. In addition, the increasein utilization factor of the catalyst can reduce a catalyst carryingamount, so that a production cost can be reduced.

In addition, the present invention can provide, at a low cost, a polymerelectrolyte fuel cell having stable electricity generation property byusing the above catalyst with improved dissipation property of theproduced water and an increased utilization factor of the catalyst(hereinafter referred to as “hydrophobic catalyst”). Furthermore, amethod of producing a catalyst layer of the present invention canrealize a catalyst layer for a polymer electrolyte fuel cell at a lowcost through an easy, inexpensive, and highly reproducible step.

According to the present invention, there can be provided a hydrophobiccatalyst layer for a polymer electrolyte fuel cell which has achievedcompatibility between an improvement in dissipation property of producedwater and an increase in utilization factor of a catalyst in thecatalyst layer.

In addition, the present invention can provide, at a low cost, a polymerelectrolyte fuel cell having stable electricity generation property byusing the above hydrophobic catalyst layer to which hydrophobicity isimparted.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing the single cell structure of apolymer electrolyte fuel cell using a hydrophobic catalyst layer of thepresent invention.

FIG. 2 is a schematic view showing an example of a hydrophobic catalystin the hydrophobic catalyst layer of the present invention.

FIG. 3 is a schematic view showing an apparatus for evaluating a polymerelectrolyte fuel cell.

FIG. 4 is a scanning electron micrograph (at a magnification of 2,500)of the surface of a hydrophobic catalyst layer of Example 1 of thepresent invention.

FIG. 5 is a scanning electron micrograph (at a magnification of 20,000)of the surface of the hydrophobic catalyst layer of Example 1 of thepresent invention.

FIG. 6 is a scanning electron micrograph (at a magnification of 100,000)of the surface of the hydrophobic catalyst layer of Example 1 of thepresent invention.

FIG. 7 is a graph showing characteristics of polymer electrolyte fuelcells of Example 1 and Comparative Example 1 of the present invention.

FIG. 8 is a graph showing changes with the elapse of time in voltages ofthe polymer electrolyte fuel cells of Example 1 and Comparative Example1 of the present invention at an output current density of 600 mA/cm².

FIG. 9 is a graph showing characteristics of polymer electrolyte fuelcells of Example 2 and Comparative Example 1 of the present invention.

FIG. 10 is a graph showing changes with the elapse of time in voltagesof the polymer electrolyte fuel cells of Example 2 and ComparativeExample 1 of the present invention at an output current density of 500mA/cm².

FIG. 11 is a graph showing characteristics of polymer electrolyte fuelcells of Example 3 and Comparative Examples 2 to 5 and 7 and 8 of thepresent invention.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, an embodiment of a hydrophobic catalyst layer for a polymerelectrolyte fuel cell according to the present invention will be shownand described with reference to the drawings. However, materials,dimensions, shapes, arrangement, and the like described in thisembodiment do not limit the scope of the present invention unlessotherwise specifically stated. The same is applied to a productionmethod to be described later.

FIG. 1 is a schematic view showing an example of the sectionalconstitution of a single cell of the fuel cell using the hydrophobiccatalyst layer for a polymer electrolyte fuel cell according to thepresent invention (hereinafter abbreviated as “hydrophobic catalystlayer”). In FIG. 1, reference numeral 1 denotes a solid polymericelectrolyte membrane. A pair of catalyst layers, that is, a catalystlayer 2 on an anode side and a catalyst layer 3 on a cathode side arearranged so that the solid polymeric electrolyte membrane 1 issandwiched between them.

In this example, a case in which the hydrophobic catalyst layer of thepresent invention is arranged only on a cathode (air electrode) side isshown. However, the arrangement and constitution of the catalyst layerare not limited to this case. For example, the hydrophobic catalystlayer of the present invention may be arranged on each of bothelectrodes, or may be arranged only on an anode side. In view of thefact that flooding is apt to occur in a catalyst layer on a cathode sidewhere water is produced, the hydrophobic catalyst layer of the presentinvention is preferably arranged at least on the cathode side.

The catalyst layer 3 on the cathode side is constituted by a hydrophobiccatalyst 4 and a catalyst carrier 5 for supporting the hydrophobiccatalyst 4. A gas-diffusion layer 7 on the cathode side and an electrode(air electrode) 9 on the cathode side are arranged outside the catalystlayer 3 on the cathode side.

A gas-diffusion layer 6 on the anode side and an electrode (fuelelectrode) 8 on the anode side are arranged outside the catalyst layer 2on the anode side.

A perfluorocarbon polymer having a sulfonic group can be suitably usedas the solid polymeric electrolyte membrane 1.

An example of a perfluorosulfonic acid polymer is Nafion (registeredtrademark, manufactured by DuPont).

When a proton H⁺ moves in an electrolyte membrane toward a cathode side,the proton moves along a hydrophilic part in the electrolyte membranewhile using a water molecule as a medium in many cases. Accordingly, theelectrolyte membrane preferably has a function of holding a watermolecule.

The solid polymeric electrolyte membrane preferably has a function ofpreventing unreacted reactant gases (hydrogen and oxygen) from passingwhile transmitting a proton H⁺ produced on the anode side toward thecathode side and a predetermined function of holding water. An arbitrarymaterial can be selected from materials each having such functions inconsideration of various conditions, and can be used in the solidpolymeric electrolyte membrane.

The gas-diffusion layers 6 and 7 each preferably have the followingfunctions: a function of uniformly and sufficiently supplying a fuel gasor the air to an electrode reaction region in a catalyst layer of a fuelelectrode or an air electrode in such a manner that an electrodereaction can be efficiently performed in plane, a function of releasingcharges generated by the electrode reaction to the outside of the singlecell, and a function of efficiently discharging water produced as aresult of a reaction and an unreacted gas to the outside of the singlecell. A porous material having electron conductivity such as carboncloth or carbon paper can be preferably used as the gas-diffusion layer.

Examples of a role of the catalyst carrier 5 to be expected include: aco-catalyst for improving catalytic activity; the maintenance of theform of the hydrophobic catalyst 4; the securement of an electronconduction channel; and an increase in specific surface area. Forexample, a carbon black layer or a gold fine particle layer can bepreferably used as the catalyst carrier.

Next, FIG. 2 schematically shows the structure of the hydrophobiccatalyst 4. The hydrophobic catalyst 4 is composed of a catalyst 11, ahydrophobic agent 12, and an electrolyte 13. The hydrophobic agent 12also enters a micro-gap 14 in the catalyst layer.

In a preferable production method of the present invention, after thehydrophobic agent 12 is formed on the catalyst 11, the electrolyte 13 isformed. Therefore, the electrolyte 13 covers the hydrophobic agent 12 insome positions as shown in FIG. 2.

Here, the amount of the hydrophobic agent 12 is such that a ratio of thenumber of Si atoms in the hydrophobic agent 12 to the number of Pt atomsin the catalyst 11 is in the range of preferably 0.15 or more and 0.25or less, or more preferably 0.18 or more and 0.22 or less.

When the amount of the hydrophobic agent 12 is excessively large, theperformance of the catalyst layer reduces because (1) most of the poresof the catalyst layer are clogged so that gas diffusion property reducesand (2) the surface of the catalyst is excessively covered with thehydrophobic agent 12 so that a contact area between the electrolyte andthe catalyst reduces.

In contrast, when the amount of the hydrophobic agent 12 is excessivelysmall, the catalyst layer cannot obtain sufficient hydrophobicity.

The catalyst 11 is composed of the aggregate of platinum nano-particlesobtained by reducing a platinum oxide, and has a dendritic shape.

The term “dendritic” as used herein refers to a structure in which alarge number of flaky tissues each constituted as a result of theaggregation of catalyst particles aggregate together while having branchpoints.

One flaky tissue preferably has a length in its shorter-side directionin the range of 5 nm or more and 200 nm or less. The term “length in ashorter-side direction” as used herein refers to the minimum dimensionin the surface of one flake. With regard to the aggregate of platinumnano-particles having a “dendritic” shape, the technique disclosed inJapanese Patent Application Laid-Open No. 2006-49278 is applicable tothe present invention.

The platinum nano-particles each preferably have a diameter of about 3to 20 nm because of high catalytic activity, and each particularlypreferably have a diameter of 3 to 10 nm because of a large surfacearea.

When the diameter of each of the platinum nano-particles is 20 nm ormore, catalytic activity reduces, whereby the performance of a fuel cellmay reduce.

A method of producing a hydrophobic catalyst layer of the presentinvention is characterized by including the steps of: bringing an Sicompound containing a hydrophobic substituent that causes a hydrolyticreaction by the catalytic action of a platinum oxide to form apolymerizable group into contact with the platinum oxide; subjecting theSi compound to hydrolysis and a polymerization reaction in the vicinityof the platinum oxide to from a hydrophobic agent on the surface of theplatinum oxide; and then reducing the platinum oxide.

It is not preferable to perform the step of bringing the Si compoundinto contact with the platinum oxide after the step of reducing theplatinum oxide. When platinum after reduction and the Si compound arebrought into contact with each other, a hydrolytic reaction proceeds ata high reaction rate, whereby an excessive amount of alkylsiloxane isformed in the catalyst layer to degrade adhesiveness with an electrolytemembrane or to clog pores in the catalyst layer.

In addition, the step of bringing the Si compound into contact with theplatinum oxide is preferably conducted for 3 to 30 minutes. When thecontact time of the Si compound in contact with the platinum oxide isexcessively short, a sufficient effect of the contact may not beobtained. In contrast, when the contact time of the Si compound incontact with the platinum oxide is excessively long, an excessive amountof alkylsiloxane is formed to degrade adhesiveness with an electrolytemembrane or to clog pores in the catalyst layer, so that the output of afuel cell reduces in some cases.

Platinum dioxide or a mixture of metal oxides mainly composed ofplatinum dioxide is more preferably used as the platinum oxide for usein the above-described step because the catalyst 11 is of a dendriticshape and the porosity of the catalyst layer increases.

In addition, the term “platinum dioxide” as used herein includes notonly one represented by a chemical formula PtO₂ but also one representedby a chemical formula PtO_(x) (X>2). Even when one represented by thechemical formula PtO_(x) (X>2) is used, an effect of the presentinvention can be obtained in the production method of the presentinvention.

Examples of the hydrophobic substituent to be used in the presentinvention include alkyl groups (the carbon chain of each of which may bebranched or may have a double bond, and a hydrogen atom of each of whichmay be substituted by a halogen atom). A methyl group can beparticularly preferably used.

In addition, the Si compound containing the hydrophobic substituent ispreferably a compound selected from the group consisting of2,4,6,8-tetraalkyl-cyclotetrasiloxane, 1,1,1,3,3,3-hexaalkyldisilazane,monoalkylsilane, dialkylsilane, and trialkylsilane, or a mixturethereof.

It is generally known that the contact of the above Si compound with ametal or the like causes a hydrolytic reaction to form Si—OH groups,whereby a dehydration condensation polymerization reaction between theSi—OH groups occurs to form a siloxane polymer having an Si atom, an Oatom, and a hydrophobic substituent. Here, an alkylsiloxane polymer isproduced when the hydrophobic substituent in the Si compound is an alkylgroup.

The above hydrolytic reaction is known to occur owing to contact with ametal, and also hydrolysis and a polymerization reaction proceed evenowing to contact with a platinum oxide.

The production method of the present invention utilizes the phenomenon.The contact of a platinum oxide with the Si compound for an appropriatetime period can produce an appropriate amount of an alkylsiloxanepolymer in the catalyst layer, whereby hydrophobicity can be effectivelyimparted.

When the hydrolytic reaction of the Si compound is caused by contactwith platinum, there is a high possibility that a reaction rate becomesso high that an excessive amount of an alkylsiloxane polymer is producedin a short time period to degrade adhesiveness with an electrolytemembrane or to clog pores in the catalyst layer.

Accordingly, it is strongly recommended that the step of bringing the Sicompound into contact with the platinum oxide be performed prior to thestep of reducing the platinum oxide.

In addition, when 1,1,1,3,3,3-hexaalkyldisilazane or trialkylsilane isused alone, since the number of polymerizable groups in one molecule issmall, a part of hydrophobic substituents is preferably hydrolyzed toform Si—OH groups by substitution as means of an approach such asirradiation with UV so that a polymerization reaction can be promoted.

A dehydration condensation polymerization reaction between Si—OH groupsproceeds even at room temperature, but an operation for heating thecatalyst layer is more preferably added after the formation of thehydrophobic agent. In such case, heating can polymerize unpolymerizedSi—OH groups in the hydrophobic agent, whereby hydrophobicity can befurther improved.

The temperature at the time of a heating treatment is preferably suchthat none of the hydrophobic substituent and any material in thecatalyst layer undergoes heat decomposition, and is more preferably 200°C. or lower.

In general, it is also important to increase the utilization efficiencyof a catalyst as well as hydrophobicity in order to obtain a catalystlayer having high performance. The hydrophobic catalyst 4 of the presentinvention is characterized in that a proton adsorption area in thesurface of the catalyst, that is, an effective surface area is largerthan that in the case where no hydrophobic treatment is performed, sothat the utilization factor of the catalyst is high.

To obtain the above characteristic, a proton conductive electrolyte ispreferably formed in the catalyst layer by adding, for example, aperfluorosulfonic acid polymer solution after the formation of thehydrophobic agent. The proton conductive electrolyte is more preferablyformed after the reduction of the platinum oxide.

The above procedure improves wettability between the hydrophobic portionof an electrolyte molecule and the hydrophobic agent. This case ispreferable because wettability between a part of the surface of thecatalyst in no contact with the hydrophobic agent and the hydrophilicportion of a proton conductive electrolyte molecule relatively improves,so that an effective surface area in the hydrophobic catalyst 4 islarger than that of a catalyst not subjected to any hydrophobictreatment.

Examples of the method of producing the hydrophobic catalyst layer ofthe present invention include various methods. An example of the methodwill be described below by taking the case of the constitution shown inFIG. 1 as an example. It should be noted that the present invention isnot limited to the following production method at all.

(1) Prepare Catalyst Layer on Cathode Side

After Au serving as a catalyst carrier has been formed into a film bymeans of an electron beam evaporation method on apolytetrafluoroethylene (PTFE) sheet as a layer to be transferred onto asolid polymeric electrolyte membrane, a porous platinum oxide catalystlayer is formed by means of a reactive sputtering method.

(2) Subject Catalyst Layer to Hydrophobic Treatment

The catalyst layer obtained in the above item (1) is brought intocontact with the gas of an Si compound containing a hydrophobicsubstituent, whereby a hydrophobic agent is formed on the surface of acatalyst. After that, the polymerization reaction of the hydrophobicagent may be promoted by heating.

Subsequently, the platinum oxide layer is subjected to a hydrogenreduction treatment, whereby a porous platinum/gold catalyst layer isobtained. After that, an appropriate amount of a solution of Nafionserving as a proton conductive electrolyte in IPA (5 wt. %, manufacturedby Wako Pure Chemical Industries, Ltd.) is dropped onto the formedcatalyst layer. After that, the solvent is volatilized in a vacuum,whereby a proton path is formed on the surface of the catalyst.

(3) Prepare Catalyst Layer on Anode Side

A catalyst layer of platinum on carbon support is formed on a PTFE sheetby using a doctor blade in the same manner as in the above item (1). Thethickness of the catalyst layer is preferably in the range of 20 to 40μm.

A catalyst slurry to be used here is a kneaded product of platinum oncarbon support (HiSPEC 4000 manufactured by Jhonson Matthey), Nafion,PTFE, isopropyl alcohol (IPA), and water.

(4) A solid polymeric electrolyte membrane (Nafion 112 manufactured byDuPont) is sandwiched between the pair of catalyst layers produced inthe foregoing such that the PTFE sheets face outward, and the sandwichedbody is subjected to hot pressing. Furthermore, the PTFE sheets arepeeled, whereby the pair of catalyst layers is transferred onto thesolid polymeric electrolyte membrane. Then, the electrolyte membrane andthe pair of catalyst layers are assembled to obtain a membrane electrodeassembly (hereinafter abbreviated as “MEA”).

(5) The MEA is sandwiched by carbon cloth (LT 1400-W manufactured byE-TEK) serving as a gas baking layer, and further by a fuel electrodeand an air electrode, whereby a single cell is produced.

The method of producing the catalyst layer of the present invention isapplicable to not only the above polymer electrolyte fuel cell having asingle cell constitution but also a polymer electrolyte fuel cellconstituted by stacking multiple single cells.

EXAMPLES

Next, the present invention will be described in detail by way ofspecific examples.

Example 1

In this example, a polymer electrolyte fuel cell having the constitutionshown in FIG. 1 as the embodiment of the present invention was produced.

Hereinafter, the production steps of the polymer electrolyte fuel cellaccording to this example will be described in detail.

(Step 1)

A gold thin film having a thickness of 50 nm was formed by means of anelectron beam vacuum evaporation method on a PTFE sheet (NITFLONmanufactured by NITTO DENKO CORPORATION) as a layer to be transferredonto a polymeric electrolyte membrane. A porous platinum oxide layerhaving a thickness of 2 μm was formed thereon by means of a reactivesputtering method. The reactive sputtering was performed under theconditions of: a total pressure of 5 Pa; an oxygen flow rate ratio(Q_(O2)/(Q_(Ar)+Q_(O2)) ) of 70%; a substrate temperature of 25° C.; andan RF input power of 5.4 W/cm².

(Step 2)

Subsequently, the porous platinum oxide layer was brought into contactwith the steam of 2,4,6,8-tetramethylcyclotetrasiloxane (hereinafterabbreviated as “TMCTS”) (having a partial pressure of 0.05 Pa) at 25° C.for 30 minutes, whereby a methylsiloxane polymer was produced on thesurface of platinum oxide. After that, it was subjected to a heattreatment in the atmosphere at 180° C. for 3 hours, whereby thecondensation polymerization of unpolymerized Si—OH groups was promoted.

(Step 3)

Subsequently, the obtained catalyst layer was subjected to a reductiontreatment in a 2% H₂/He atmosphere at 0.1 MPa for 30 minutes, whereby aporous platinum catalyst layer was obtained on the PTFE sheet. The Ptcarrying amount in this case was 0.85 mg/cm². The equilibrium contactangle of the catalyst layer with respect to water at this time was 138°,and the surface of the catalyst layer was hydrophobic.

In addition, a scanning electron microscope was used to observe thatmethylsiloxane polymers were present on the catalyst layer as shown ineach of FIGS. 4 to 6. In each of FIGS. 4 to 6, a dark spot-like part isa methylsiloxane polymer produced on a catalyst.

As can be seen from FIG. 6, a methylsiloxane polymer enters even amicro-gap having a size of 100 nm or less in the catalyst layer.

The methylsiloxane polymer shown in FIG. 6 is a relatively large polymerin the entire layer which is zoomed in for aiding the understanding ofthe gist of the present invention. A large number of polymers eachhaving a size of several tens of nanometers and smaller than the aboverelatively large polymer were also present in the catalyst layer.

In addition, a part other than a dark spot-like part shown in FIG. 5shows a dendritic-shaped catalyst, that is, the catalyst has a shape ofaggregate of a large number of flaky tissues having branch points.Observation with a transmission electron microscope (TEM) confirmed thateach of the flaky parts was the aggregate of platinum fine particleseach having a diameter of about 5 to 10 nm.

A ratio Si/Pt of the number of Si atoms in the obtained catalyst layerto the number of Pt atoms in the layer measured by using a scanningfluorescent X-ray analyzer (ZSX 100e manufactured by Rigaku Corporation)was 0.22.

After that, a 5-wt. % Nafion solution (manufactured by Wako PureChemical Industries, Ltd.) was dropped to the obtained catalyst layer inan amount of 8 μl per 1 cm² of a catalyst area, and the solvent wasvolatilized in a vacuum, whereby a proton path was formed on the surfaceof the catalyst.

(Step 4)

In this step, a catalyst layer of platinum on carbon support wasproduced as a catalyst layer to form a pair with the catalyst layerproduced in (Step 3) described above. The catalyst layer of platinum oncarbon support was formed on a PTFE sheet as a layer to be transferredonto a solid polymeric electrolyte membrane by using a doctor blade. Acatalyst slurry used here was a kneaded product of platinum on carbonsupport (HiSPEC 4000 manufactured by Jhonson Matthey), Nafion, IPA, andwater. The Pt carrying amount in this case was 0.35 mg/cm².

(Step 5)

A solid polymeric electrolyte membrane (Nafion 112 manufactured byDuPont) was sandwiched between the two catalyst layers produced in (Step3) and (Step 4) described above, and they were subjected to hot pressingunder the pressing conditions of 8 MPa, 150° C., and 1 min. The PTFEsheets were peeled, whereby the pair of catalyst layers was transferredonto the solid polymeric electrolyte membrane. Then, the electrolytemembrane and the pair of catalyst layers were assembled each other.

(Step 6)

The assembly including the hydrophobic catalyst layer of the presentinvention on a cathode side and the catalyst layer of platinum on carbonsupport on an anode side was covered with carbon clothes (LT-1400Wmanufactured by E-TEK) serving as a gas baking layer, and further with afuel electrode and an air electrode in such order as shown in FIG. 1,whereby a single cell was formed.

The single cell produced through the above steps was evaluated forcharacteristics by using an evaluation apparatus having the constitutionshown in FIG. 3. An electrical discharge test was performed at a celltemperature of 80° C. while the anode electrode side was filled with ahydrogen gas in a dead end manner and the cathode electrode side wasopened to the air. As a result, current-voltage characteristics shown inFIG. 9 were obtained.

Comparative Example 1

FIG. 7 shows, as Comparative Example 1, an example using a catalystlayer produced in the same manner as in Example 1 except that (Step 2)described above was omitted. The Pt carrying amount of the catalystlayer was the same as that of Example 1, that is, 0.85 mg/cm². Inaddition, the equilibrium contact angle of the catalyst layer ofComparative Example 1 with respect to water was 6.3°, and the surface ofthe catalyst layer was hydrophilic. In addition, the Si/Pt ratio ofComparative Example 1 was zero because methylsiloxane was not appliedthereto.

First, comparison between current densities at 0.9 V as a reactionrate-determining region confirmed that the current density of Example 1was 12.3 mA/cm² though the current density of Comparative Example 1 was7.6 mA/cm². Furthermore, comparison between catalytic specificactivities each obtained by dividing a current density by a Pt carryingamount confirmed that the catalytic specific activity of Example 1 was14.5 A/g though the catalytic specific activity of Comparative Example 1was 8.9 A/g.

That is, the deterioration of cell characteristics in the catalyst layerof Example 1 due to activation polarization was significantly suppressedas compared to that in the catalyst layer of Comparative Example 1. Thisresult shows that the methylsiloxane polymer of Example 1 does notinhibit an oxidation-reduction reaction on the surface of the catalyst,or rather improves the activity of the catalyst layer. This is probablydue to an increase in effective surface area of the catalyst layer asdescribed later.

In addition, comparison between voltages at 600 mA/cm² as a diffusionpolarization rate-determining region confirmed that a voltage of 0.42 Vor more was taken from the single cell of Example 1 but only about 0.3 Vwas taken from the single cell of Comparative Example 1. That is, thedeterioration of cell characteristics in the catalyst layer of Example 1due to diffusion polarization was significantly suppressed as comparedto that in the catalyst layer of Comparative Example 1. This shows thatthe hydrophobic catalyst layer of Example 1 is superior to the catalystlayer of Comparative Example 1 in dissipation property of producedwater.

Next, FIG. 8 shows a change with the elapse of time in voltage when thesingle cell of Example 1 was caused to generate electricity continuouslyat a current density of 600 mA/cm² together with the result of thesingle cell of Comparative Example 1.

The single cell using the hydrophobic catalyst layer of Example 1 had avoltage of 0.3 V or more even after the lapse of 1 hour and 40 minutes.In contrast, the voltage of the single cell of Comparative Example 1became zero in 12 minutes, and electricity generation stopped.

This shows that the hydrophobic catalyst layer of Example 1significantly improved the stability of the performance of a fuel cellbecause the layer was superior to the catalyst layer of ComparativeExample 1 in dissipation property of produced water.

Next, cyclic voltammogram measurement was performed at a celltemperature of 80° C. while a hydrogen gas was flowed at 20 sccm to theanode electrode side and an N₂ gas was flowed at 40 sccm to the cathodeelectrode side, whereby an H⁺ adsorption area per unit electrode area,that is, an effective surface area was measured.

While the effective surface area of the catalyst layer of Example 1 perunit area of the electrode was 282 cm², the effective surface area ofthe catalyst layer of Comparative Example 1 was 208 cm². Although thehydrophobic catalyst layer of Example 1 and the catalyst layer ofComparative Example 1 had the same platinum carrying amount, theeffective surface area of the hydrophobic catalyst layer of Example 1increased as compared to that of the catalyst layer of ComparativeExample 1 by 30% or more, so that the utilization factor of the catalystsignificantly increased.

Example 2

In this example, the constitution of a polymer electrolyte fuel cellshown in FIG. 1 as the embodiment of the present invention was producedby using the catalyst layer of the present invention and a method ofproducing the catalyst layer.

Hereinafter, the production steps of the polymer electrolyte fuel cellaccording to this example will be described in detail.

(Step 1)

A gold thin film having a thickness of 50 nm was formed by means of anelectron beam vacuum evaporation method on a PTFE sheet (NITFLONmanufactured by NITTO DENKO CORPORATION) as a layer to be transferredonto a solid polymeric electrolyte membrane. A porous platinum oxidelayer having a thickness of 2 μm was formed thereon by means of areactive sputtering method. The reactive sputtering was performed underthe conditions of: a total pressure of 5 Pa; an oxygen flow rate ratio(Q_(O2)/(Q_(Ar)+Q_(O2)) ) of 70%; a substrate temperature of 25° C.; andan RF input power of 5.4 W/cm².

(Step 2)

Subsequently, the porous platinum oxide layer was brought into contactwith the steam of 1,1,1,3,3,3-hexamethyldisilazane (having a partialpressure of 105 hPa) at 50° C. for 10 minutes under irradiation with anultraviolet, whereby the film of a methylsiloxane polymer was formed onthe surface of a platinum oxide. After that, it was subjected to a heattreatment in the atmosphere at 180° C. for 3 hours, whereby thecondensation polymerization of unpolymerized Si-OH groups was promoted.

The subsequent steps ((Step 3) to (Step 6)) were performed in the samemanner as in Example 1, whereby a single cell was formed. The Ptcarrying amount in this example was 0.85 mg/cm². In addition, theequilibrium contact angle of the catalyst layer with respect to waterwas 138°, and the surface of the catalyst layer was hydrophobic. Inaddition, the Si/Pt ratio in this example was 0.18.

The single cell produced through the above steps was evaluated forcharacteristics by using an evaluation apparatus having the constitutionshown in FIG. 3. An electrical discharge test was performed at a celltemperature of 80° C. while a hydrogen gas was flowed to the anodeelectrode side and the air was flowed to the cathode electrode side. Asa result, current-voltage characteristics shown in FIG. 9 were obtained.

First, comparison between current densities at 0.9 V as a reactionrate-determining region confirmed that the current density of Example 2was 14.9 mA/cm₂ though the current density of Comparative Example 1 was7.6 mA/cm². Furthermore, comparison between catalytic specificactivities each obtained by dividing a current density by a Pt carryingamount confirmed that the catalytic specific activity of Example 2 was17.5 A/g though the catalytic specific activity of Comparative Example 1was 8.9 A/g.

That is, the deterioration of cell characteristics in the catalyst layerof Example 2 due to activation polarization was significantly suppressedas compared to that in the catalyst layer of Comparative Example 1. Thisresult shows that the methylsiloxane polymer of Example 2 does notinhibit an oxidation-reduction reaction on the surface of the catalyst,or rather improves the activity of the catalyst.

In addition, comparison between voltages at 500 mA/cm² as a diffusionpolarization rate-determining region confirmed that a voltage of 0.53 Vwas taken from the single cell of Example 2 but only 0.4 V or less wastaken from the single cell of Comparative Example 1. That is, thedeterioration of cell characteristics in the catalyst layer of Example 2due to diffusion polarization was significantly suppressed as comparedto that in the catalyst layer of Comparative Example 1. This shows thatthe hydrophobic catalyst layer of Example 2 is superior to the catalystlayer of Comparative Example 1 in dissipation property of producedwater.

Next, FIG. 10 shows a change with the elapse of time in voltage when thesingle cell of Example 2 was caused to generate electricity continuouslyat a current density of 500 mA/cm² together with the result of thesingle cell of Comparative Example 1.

The single cell using the hydrophobic catalyst layer of Example 2 had avoltage of 0.48 V even after the lapse of 50 minutes. In contrast, thevoltage of the single cell of Comparative Example 1 became zero in about27 minutes, and electricity generation stopped. This shows that thehydrophobic catalyst layer of the present invention significantlyimproved the stability of the performance of a fuel cell because thelayer was superior to the catalyst layer of Comparative Example 1 indissipation property of produced water.

Next, cyclic voltammogram measurement was performed at a celltemperature of 80° C. while a hydrogen gas was flowed at 20 sccm to theanode electrode side and an N₂ gas was flowed at 40 sccm to the cathodeelectrode side, whereby an effective surface area was measured.

While the effective surface area of the catalyst layer of Example 1 perunit area of the electrode was 236 cm², the effective surface area ofthe catalyst layer of Comparative Example 1 was 208 cm². The effectivesurface area of the hydrophobic catalyst of Example 2 increased by 13%or more, as compared to that in the case where a catalyst not subjectedto any hydrophobic treatment was used, so that the utilization ratio ofthe catalyst significantly increased.

Example 3

(Step 1)

A porous platinum oxide layer having a thickness of 2 μm was formed bymeans of a reactive sputtering method on a surface composed of carbonfine particles of carbon cloth (LT-1400W manufactured by E-TEK) as asubstrate for a catalyst layer serving also as a gas-diffusion layer.The reactive sputtering was performed under the conditions of: a totalpressure of 5 Pa; an oxygen flow rate ratio (Q_(O2)/(Q_(Ar)+Q_(O2))) of70%; a substrate temperature of 25° C.; and an RF input power of 5.4W/cm².

(Step 2)

Subsequently, the composite of the porous platinum oxide layer and thegas-diffusion layer was brought into contact with the steam of TMCTS(having a partial pressure of 0.05 Pa) at 25° C. for 5 minutes, wherebya methylsiloxane polymer was formed on the surface of a platinum oxide.In Example 3, a heat treatment like Example 1 was not performed as asubsequent step.

(Step 3)

Subsequently, the obtained catalyst layer was subjected to a reductiontreatment in a 2% H₂/He atmosphere at 0.1 MPa for 30 minutes, whereby aporous platinum catalyst layer-gas baking layer composite was obtained.The Pt carrying amount in this case was 0.85 mg/cm². The equilibriumcontact angle of the catalyst layer with respect to water at this timewas 131°, and the surface of the catalyst layer was hydrophobic.

After that, a 5-wt % Nafion solution (manufactured by Wako Pure ChemicalIndustries, Ltd.) was dropped to the obtained catalyst layer in anamount of 8 μl per 1 cm² of a catalyst area, and the solvent wasvolatilized in a vacuum, whereby a proton path was formed on the surfaceof the catalyst.

(Step 4)

A solid polymeric electrolyte membrane (Nafion 112 manufactured byDuPont) was sandwiched between the hydrophobic catalyst layer producedin (Step 3) described above and the catalyst layer of platinum on carbonsupport obtained in (Step 4) of Example 1, and they were subjected tohot pressing under the pressing conditions of 4 MPa, 150° C., and 20min. The PTFE sheet on the side of the catalyst layer of platinum oncarbon support was peeled, whereby the pair of catalyst layers wastransferred onto the solid polymeric electrolyte membrane. Thus, an MEAintegrated with the gas baking layer was obtained.

The subsequent steps ((Step 5) and (Step 6)) were performed in the samemanner as in Example 1, whereby a single cell was formed.

The single cell produced through the above steps was subjected to anelectrical discharge test in the same manner as in Example 1. FIG. 11shows the result.

In addition, comparative examples for Example 3 are described below.FIG. 11 shows the current-voltage characteristics of each comparativeexample. In addition, for easy comparison, Table 1 shows the currentdensity at 0.9 V, catalytic specific activity, critical current value,effective surface area, and ratio Si/Pt of the number of Si atoms to thenumber of Pt atoms of each of Example 3 and comparative examples.

Comparative Example 2

A single cell was formed by using a catalyst layer produced in the samemanner as in Example 3 except that (Step 2) was omitted. The Pt carryingamount of the catalyst layer was the same as that of Example 3, that is,0.84 mg/cm². In addition, the equilibrium contact angle of the catalystlayer of Comparative Example 2 with respect to water was 6.3°, and thesurface of the catalyst layer was hydrophilic.

Comparative Example 3

A single cell was formed by using a catalyst layer produced in the samemanner as in Example 3 except that, in (Step 2), instead of beingbrought into contact with the steam of TMCTS, the porous platinum oxidelayer was immersed in a tetrafluoroethylene (PTFE) dispersion solution(Polyfron, 60 wt. %, manufactured by DAIKIN INDUSTRIES, ltd., averageparticle size 300 μm) diluted with pure water to have a concentration of20%, and was then lifted and air-dried at room temperature.

The Pt carrying amount in this case was 0.84 mg/cm². The equilibriumcontact angle of the catalyst layer with respect to water was 146°, andthe surface of the catalyst layer was hydrophobic.

Comparative Example 4

A single cell was formed by using a catalyst layer produced in the samemanner as in Example 3 except that, in (Step 2), the porous platinumoxide layer was brought into contact with the steam of TMCTS (having apartial pressure of 0.05 Pa) at 25° C. for 1 minute.

The Pt carrying amount in this case was 0.84 mg/cm². The equilibriumcontact angle of the catalyst layer with respect to water was 20°, andthe surface of the catalyst layer was hydrophilic.

Comparative Example 5

A single cell was formed by using a catalyst layer produced in the samemanner as in Example 3 except that, in (Step 2), the porous platinumoxide layer was brought into contact with the steam of TMCTS (having apartial pressure of 0.05 Pa) at 25° C. for 60 minutes.

The Pt carrying amount in this case was 0.84 mg/cm². The equilibriumcontact angle of the catalyst layer with respect to water was 138°, andthe surface of the catalyst layer was hydrophobic.

Comparative Example 6

A catalyst layer was produced in the same manner as in Example 3 exceptthat: (Step 2) was performed after the hydrogen reduction treatment of(Step 3); and, in (Step 2), the porous platinum oxide layer was broughtinto contact with the steam of TMCTS (having a partial pressure of 0.05Pa) at 25° C. for 3 minutes.

The catalyst layer was subjected to hot pressing against a solidpolymeric electrolyte membrane (Nafion 112) in the same manner as in(Step 4) of Example 3. As a result, at a large number of positions thecatalyst layer could not be transferred onto Nafion 112 occurred, and anMEA could not be formed.

Comparative Example 7

A single cell was formed by using a catalyst layer produced in the samemanner as in Example 3 except that: the thickness of the platinum oxidelayer was set to about 1.8 μm in (Step 1); (Step 2) of Example 3 wasperformed after the hot pressing of (Step 5) (the order of steps waschanged); and, in (Step 2), the porous platinum oxide layer was broughtinto contact with the steam of TMCTS (having a partial pressure of 0.02Pa) at 4° C. for 3 minutes.

The Pt carrying amount in this case was 0.71 mg/cm². The equilibriumcontact angle of the catalyst layer with respect to water was 138°, andthe surface of the catalyst layer was hydrophobic.

Comparative Example 8

A single cell was formed by using a catalyst layer produced in the samemanner as in Example 3 except that: the thickness of the platinum oxidelayer was set to about 3 μm in (Step 1); (Step 2) was performed afterthe hot pressing of (Step 5) (the order of steps was changed); and, in(Step 2), the porous platinum oxide layer was brought into contact withthe steam of TMCTS (having a partial pressure of 0.05 Pa) at 25° C. for6 minutes.

The Pt carrying amount in this case was 1.1 mg/cm². The equilibriumcontact angle of the catalyst layer with respect to water was 138°, andthe surface of the catalyst layer was hydrophobic.

FIG. 11 and Table 1 show the results of Example 3 and ComparativeExamples 2 to 5 and 7 and 8.

The effective surface area of each of Comparative Examples 7 and 8 isnot measured because it is judged to be difficult to compare theeffective surface area of each of Comparative Examples 7 and 8 with thatof Example 3 owing to a large difference in Pt carrying amount betweeneach of Comparative Examples 7 and 8, and Example 3. In addition, no Siatom was observed in each of Comparative Examples 2 and 3 becausemethylsiloxane was not added thereto. TABLE 1 Effective surface area perCurrent Catalytic Critical unit area density specific current of at 0.9V activity density Si/Pt electrode (mA/cm²) (A/g) (mA/cm²) ratio (cm²)Example 3 11.5 13.6 720.5 0.21 240.1 Comparative 7.2 8.6 631.3 0.00205.4 Example 2 Comparative 4.0 4.8 770.0 0.00 190.0 Example 3Comparative 10.4 12.3 645.7 0.14 209.4 Example 4 Comparative 6.4 7.6242.5 0.36 177.4 Example 5 Comparative 3.3 4.6 188.4 0.30 Example 7Comparative 7.8 7.1 389.6 0.34 Example 8

As can be seen from Example 3 and Table 1, Example 3 had the largestspecific activity, and provided a voltage value higher than that of anyother comparative examples over a wide current density range of 100 to500 mA/cm². In addition, the critical current density of Example 3considerably increased as compared to that of Comparative Example 2.

In addition, as in the case of each of Examples 1 and 2, the specificactivity and effective surface area of Example 3 considerably increasedas compared to those of Comparative Example 2 in which no hydrophobictreatment was performed. This shows that the utilization factor of acatalyst increases.

In Comparative Example 3, a critical current density was high, butspecific activity and a voltage up to 600 mA/cm² considerably reduced ascompared to those of Example 3. This is probably because the particlesize of a PTFE fine particle was as large as several hundreds ofmicrometers, so that the hydrophobic agent was not dispersed, and hencethe catalyst layer could not be effectively made hydrophobic.

In addition, the specific activity and voltage up to 450 mA/cm² ofComparative Example 3 are smaller than those of Comparative Example 2 inwhich no hydrophobic treatment was performed.

That is, the reason why the reduced utilization factor of the catalystin Comparative Example 3 is probably as follows. Although the impartmentof hydrophobicity to the catalyst layer by a PTFE fine particle wasattained, a part in which an excessively large amount of PTFE fineparticles were present and a part in which an excessively small amountof PTFE fine particles were present appeared in the catalyst layer, andgas diffusion in each of the parts was inhibited.

The current-voltage characteristics of Comparative Example 4 slightlyimproved as compared to those of Comparative Example 2 in which nohydrophobic treatment was performed, but were inferior to those ofExample 3.

In addition, the specific activity of Comparative Example 4 improved ascompared to that of Comparative Example 2. However, the critical currentand effective surface area of Comparative Example 4 kept to becomparable to those of Comparative Example 2, and did not reach those ofExample 3.

This is probably because the time period for contact of the porousplatinum oxide layer with the steam of TMCTS was so short thatsufficient hydrophobicity could not be imparted to the catalyst layer.

The specific activity, critical current, and effective surface area ofComparative Example 5 considerably reduced as compared to those ofExample 3. This is probably because the time period for contact of theporous platinum oxide layer with the steam of TMCTS was so long that anexcessive amount of a methylsiloxane polymer was produced in thecatalyst layer.

That is, the performance of the fuel cell reduced owing to the excessiveamount of methylsiloxane probably because (1) the amount of the catalystsurface covered with methylsiloxane excessively increased so that acontact area between Nafion as an electrolyte and the catalyst reducedand (2) the pores in the catalyst layer were clogged with methylsiloxaneso that the diffusion property of an oxygen gas reduced.

The results of Examples 1 to 3 and Comparative Examples 4 and 5 showthat the Si/Pt ratio is preferably in the range of about 0.15 or moreand 0.25 or less in order to obtain a hydrophobic catalyst layer havinghigh performance by using the constitution of the present invention. Inaddition, the results show that it is important to appropriately controlthe time period for contact of the steam of TMCTS and a platinum oxidewith each other.

As described above, in Comparative Example 6, an MEA could not beproduced owing to the occurrence of a portion where the catalyst layerwas insufficiently transferred onto Nafion 112.

This is because a reaction rate in each of hydrolysis and apolymerization reaction caused by catalyst contact between TMCTS and aplatinum catalyst was higher than a reaction rate in each of hydrolysisand a polymerization reaction caused by catalyst contact between TMCTSand a platinum oxide catalyst, so that these reactions excessivelyproceeded.

That is, the adhesiveness between Nafion 112 and the catalyst layerreduced as a result of the formation of an excessive amount of amethylsiloxane polymer in the catalyst layer due to contact between aplatinum catalyst having high activity and TMCTS.

The specific activity and critical current of each of ComparativeExamples 7 and 8 considerably reduced as compared to those of Example 3.In particular, each of the current density at 0.9 V, specific activity,and critical current of Comparative Example 8 was lower than that ofExample 3 despite the fact that the Pt carrying amount of ComparativeExample 8 was larger than that of Example 3.

This is because an excessive amount of a methylsiloxane polymer wasproduced in the catalyst layer owing to contact between a platinumcatalyst having high activity and TMCTS as in the case of ComparativeExample 5.

Furthermore, in the steps of each of Comparative Examples 7 and 8, afterthe reduction of the catalyst, Nafion is added, and then TMCTS isbrought into contact with the catalyst. In this case, the hydrolysis ofTMCTS proceeds in the vicinity of a surface of Pt not covered withNafion, so that the formation of methylsiloxane centers on the vicinity.Accordingly, a so-called three-phase interface (interface whereplatinum, Nafion, and a reactant gas simultaneously contacted with oneanother) was covered with methylsiloxane, so that the area of thethree-phase interface probably considerably reduced. As a result, theperformance of the fuel cell reduced.

The results of Comparative Examples 6, 7 and 8 show that it is necessarythat a platinum oxide is subjected to a reduction treatment after thesteam of TMCTS and the platinum oxide have been brought into contactwith each other in order to obtain a hydrophobic catalyst layer havinghigh performance by using the constitution of the present invention.

As shown in the foregoing examples, the use of the hydrophobic catalystlayer according to the present invention as a catalyst layer for apolymer electrolyte fuel cell provided a fuel cell having: significantlyimproved dissipation property of produced water and a significantlyincreased utilization factor of a catalyst in the catalyst layer; andexcellent cell characteristics. Furthermore, the method of producing acatalyst layer according to the examples was able to realize a polymerelectrolyte fuel cell having stable characteristics at a low costbecause the method was an easy, inexpensive, highly reproducibleprocess.

INDUSTRIAL APPLICABILITY

The hydrophobic catalyst layer of the present invention can be used as acatalyst layer for a polymer electrolyte fuel cell because thedissipation property of produced water and the utilization factor of thecatalyst in the catalyst layer can be improved.

In addition, a polymer electrolyte fuel cell having the catalyst layercan be used as a fuel cell for a small-size electrical apparatus such asa portable phone, a notebook personal computer, or a digital camera.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2005-132957, filed on Apr. 28, 2005, which is hereby incorporated byreference herein in its entirety.

1. A hydrophobic catalyst layer for a polymer electrolyte fuel cell,comprising a catalyst, a hydrophobic agent, and a proton conductiveelectrolyte, wherein the catalyst is a dendritic-shaped catalystobtained by reducing a platinum oxide, wherein the hydrophobic agentcomprises a compound having an Si atom, an O atom, and a hydrophobicsubstituent, and wherein a ratio Si/Pt of a number of Si atoms in thehydrophobic agent to a number of Pt atoms in the catalyst is in a rangeof 0.15 or more and 0.25 or less.
 2. A hydrophobic catalyst layer for apolymer electrolyte fuel cell according to claim 1, wherein thehydrophobic agent comprises a siloxane polymer having a hydrophobicsubstituent.
 3. A hydrophobic catalyst layer for a polymer electrolytefuel cell according to claim 1, wherein the hydrophobic agent comprisesalkylsiloxane.
 4. A method of producing a hydrophobic catalyst layer fora polymer electrolyte fuel cell, comprising the steps of: bringing an Sicompound containing a hydrophobic substituent, which causes a hydrolyticreaction owing to a catalytic action of a platinum oxide to form apolymerizable group, into contact with the platinum oxide; subjectingthe Si compound to a polymerization reaction in a vicinity of theplatinum oxide to form a hydrophobic agent on a surface of the platinumoxide; and then reducing the platinum oxide.
 5. A method of producing ahydrophobic catalyst layer for a polymer electrolyte fuel cell accordingto claim 4, wherein the Si compound containing the hydrophobicsubstituent comprises at least one or more compounds selected from thegroup consisting of 2,4,6,8-tetraalkylcyclotetrasiloxane,1,1,1,3,3,3-hexaalkyl-disilazane, monoalkylsilane, dialkylsilane, andtrialkylsilane, or a mixture thereof.
 6. A polymer electrolyte fuelcell, comprising at least the hydrophobic catalyst layer according toclaim 1, and a solid polymeric electrolyte membrane.
 7. A method ofproducing a polymer electrolyte fuel cell, comprising a step of forminga hydrophobic catalyst layer by a method of producing a hydrophobiccatalyst layer for a polymer electrolyte fuel cell according to claim 4.